1. Field of the Invention
The present invention relates to an antenna having bidirectional characteristic
2. Related Art of the Invention
A conventional technique will be discussed in accordance with FIGS. 21 to 24.
FIG. 21 shows an example of an antenna having bidirectional radiation patterns on a horizontal surface, and FIG. 22 shows an example of a prototype antenna. Further, FIG. 23 shows input impedance characteristics of the prototype antenna, and FIG. 24 shows radiating directivity of the prototype antenna.
In FIG. 21, reference number 111 denotes a feeding section, reference numeral 112 denotes an antenna element, reference numeral 113 denotes a cavity, reference numeral 114 denotes a linear conductor, and reference numerals 115 and 116 denote openings. The feeding section 111 is positioned at the center of the bottom of the cavity 113, one of the ends of the antenna element 112 is connected to the feeding section 111 and the other end is electrically connected to the linear conductor 114.
The following configuration is shown as an example: the cavity 113 forms a rectangular parallelepiped symmetric with respect to ZY surface and ZX surface, the two rectangular openings 115 and 116, which are identical in form with the linear conductor 114 being sandwiched therebetween, are disposed on the upper surface of the cavity 113 so as to be symmetric with respect to ZY surface, the feeding section 111 is disposed on the origin of the XY surface, the antenna element 112 is composed of a conductor line perpendicular to XY surface, and the linear conductor 114 and the antenna element 112 are mechanically and electrically connected to each other by soldering and the like.
Here, a space surrounded by the cavity 113 is referred to as the interior of the antenna, and a space opposite to the interior of the antenna relative to the cavity 113 is referred to as the exterior of the antenna.
FIG. 22 shows the prototype antenna. As an example, the bottom of the cavity 113 is a square having a side of 0.835xc3x97xcex0  (xcex0:free space wavelength) and a height of 0.0835xc3x97xcex0  relative to a free space wavelength xcex0  of a center frequency f0. The linear conductor 114 is disposed on ZY surface in parallel with Y axis with a length of 0.835xc3x97xcex0, and both ends of the linear conductor 114 are electrically connected to the sides of the cavity 113. And the two openings 115 and 116 are rectangular, each having a side of 0.209xc3x97xcex0 in parallel with X axis and a side of 0.835xc3x97xcex0 in parallel with Y axis. The two openings 115 and 116 are disposed so as to be adjacent to each other with the linear conductor 114 being sandwiched therebetween at the center of the ceiling of the antenna. The above-mentioned antenna has a symmetric structure with respect to ZX surface and ZY surface. At this moment, the antenna element 112 is composed of a conductor line and is 0.0835xc3x97xcex0 in length. The protruding end of the antenna element 112 is electrically connected to the linear conductor 114 on the ceiling of the antenna.
FIG. 23 shows VSWR (Voltage Standing Wave Ratio) characteristics relative to a 50-ohm feeding line regarding input impedance characteristics of the prototype antenna. The lateral axis is standardized at a center frequency of f0). f1 of FIG. 23 is a minimum frequency where VSWR satisfies 2 or less. f2 is a maximum frequency where VSWR satisfies 2 or less. As shown in FIG. 23, a band where VSWR is 2 or less accounts for 18.2% in a fractional bandwidth ((f2xe2x88x92f1)/f0). It is found that fine impedance characteristics are shown over a broad band with small reflection loss.
FIG. 24 shows as an example the radiating directivity at a center frequency to regarding the antenna having the above configuration. The radiating directivity is marked in 10 dB, and the unit is dBi, which is based on radiant power of a point wave source. As shown in FIG. 24, the above-mentioned antenna suppresses radiation of radio waves in Y direction and obtains bidirectional radiation patterns in X direction. Therefore, the above example shows excellent characteristics in a narrow interior space such as a corridor.
Moreover, the antenna element 112 is 0.0835xc3x97xcex0  in height and is shorter than a typical xc2xc wavelength antenna element. As described above, according to the configuration of the above-mentioned antenna, the antenna element 112 can be smaller in height. When the antenna cannot be embedded into the ceiling of a room, it is possible to realize an antenna having a preferable appearance with a small protruding part being out of a person""s sight on the ceiling.
In addition, in the above-mentioned conventional technique, the antenna is symmetric with respect to ZY surface and ZX surface. In this case, the directivity of radiant radio waves from the antenna is symmetric with respect to ZY surface and ZX surface.
As described above, it is possible to realize a small and excellent antenna that has desired bidirectional radiation patterns with a simple configuration.
However, in the conventional example of FIG. 21 is disadvantageous as follows: although the above-described configuration can achieve broadband impedance characteristics, it is not possible to have fine impedance characteristics and bidirectional radiation patterns on a broader band. For this reason, when a frequency bandwidth used by a plurality of applications is a broad band, a plurality of antennas is necessary.
However, a wider space is necessary for setting a plurality of antennas, and a plurality of signal transmission lines is further required, which is conspicuous and is undesirable in appearance. Also, the cost is increased.
Therefore, in order to achieve preferable appearance with low cost, when a frequency bandwidth used by a plurality of applications is a broad band, the configuration of the conventional example is inevitably unsuitable because it cannot obtain bidirectional radiation patterns over a broad band.
When a frequency bandwidth used by a plurality of applications is a broad band, it is necessary to obtain fine impedance characteristics and bidirectional radiation patterns at frequencies over a band broader than that of the conventional antenna.
Hence, in view of the above-mentioned problem, the present invention aims to provide an antenna which is small in size particularly on the upper side and obtains bidirectional radiation patterns over a broad band.
One aspect of the present invention is an antenna, comprising:
a box conductive case having at least a single opening on an upper part,
an internal conductor which is stored in said case, is disposed at a bottom, and is shaped like a letter xe2x80x9cxe2x80x9d which is one of Japanese katakana letters, a letter xe2x80x9cuxe2x80x9d, a letter xe2x80x9cUxe2x80x9d, a cramp, a horseshoe or an arc, and
a feeding element which is stored in said conductive case and is connected to a feeding section disposed on said bottom of said conductive case,
wherein said internal conductor other than parts disposed on said conductive case is not connected to said case.
Another aspect of the present invention is the antenna, wherein said feeding element is connected to a ceiling of said internal conductor.
Still another aspect of the present invention is the antenna, further comprising a gap for electrically opening said feeding element from a feeding section of said internal conductor, said gap being provided between said feeding element and a ceiling of said internal conductor.
Yet still another aspect of the present invention is the antenna, further comprising at least one or more matching conductors being electrically connected to said conductive case.
Still yet another aspect of the present invention is the antenna, wherein at least one or more of said matching conductors are electrically connected to said feeding element.
A further aspect of the present invention is the antenna, wherein at least one or more of said matching conductors are electrically connected to said internal conductor.
A still further aspect of the present invention is the antenna, wherein a space including said feeding element is entirely or partially filled with a dielectric, said space being surrounded by said conductive case.
A yet further aspect of the present invention is the antenna, wherein said dielectric is a dielectric substrate,
said conductive case includes a metallic foil pattern attached on said dielectric substrate and/or a via provided on said dielectric substrate,
said internal conductor has a ceiling including said metallic foil pattern attached on said dielectric substrate, and
said internal conductor has a side including said via provided on said dielectric substrate.
A still yet further aspect of the present invention is the antenna, further comprising opening control means of adjusting a size of said opening.
An additional aspect of the present invention is the antenna, further comprising ceiling conductor adjusting means of adjusting a ceiling size of said internal conductor.
A still additional aspect of the present invention is the antenna, wherein said bottom of said conductive case is circular.
A yet additional aspect of the present invention is the antenna, wherein said bottom of said conductive case is a rectangular parallelepiped.
A still yet additional aspect or the present invention is the antenna, wherein when said internal conductor is shaped like a letter xe2x80x9cxe2x80x9d which is one of Japanese katakana letters, a length of a ceiling of said internal conductor, in a direction parallel to a direction from a part to the other part that are in contact with said conductive case is shorter than a wavelength of the highest frequency in a frequency band having better characteristics than predetermined characteristics.
A supplementary aspect of the present invention is the antenna according to 1st invention, wherein when using rectangular coordinates having an origin placed at a center of said conductive case, X axis and Y axis that are placed on said bottom of said conductive case, and Z axis intersecting said bottom, said conductive case is symmetric with respect to ZX surface and ZY surface of said rectangular coordinates, and said feeding section is placed on Y axis of said rectangular coordinates.
A still supplementary aspect of the present invention is the antenna, wherein said internal conductor has a center at said origin.
A yet supplementary aspect of the present invention is the antenna, wherein said internal conductor is symmetric with respect to said ZX surface and ZY surface.
A still yet supplementary aspect of the present invention is the antenna, wherein said X axis is along a direction of radiating an electromagnetic wave.
One aspect of the present invention is the antenna according, further comprising at least one or more directivity control conductors.
Another aspect of the present invention is the antenna, when using rectangular coordinates having an origin placed at the center of said conductive case, X axis and Y axis that are placed on said bottom of said conductive case, and Z axis intersecting said bottom, said directivity control conductors are placed so as to be symmetric with respect to ZY surface of said rectangular coordinates.
Still another aspect of the present invention is the antenna, wherein said directivity control conductors are placed so as to be symmetric with respect to ZX surface of said rectangular coordinates.
Yet still another aspect of the present invention is the antenna, wherein at least one of said directivity control conductors is connected to said conductive case.
Still yet another aspect of the present invention is the antenna, wherein a resonance frequency of said internal conductor, a resonance frequency of said conductive case on a surface in parallel with said internal conductor and perpendicular to said bottom of said conductive case, and a resonance frequency of said conductive case on a surface perpendicular to said internal conductor and said bottom of said conductive case are different from one another.
A further aspect of the present invention is the antenna, wherein said internal conductor is connected to said conductive case via a capacitor.
A still further aspect of the present invention is the antenna, wherein said internal conductor is connected to said conductive case via a coil.